Method of injecting plurality of spacecrafts into different orbits individually

ABSTRACT

Disclosed is a method for injecting a plurality of spacecrafts into different circum-earth or interplanetary orbits individually in a single launch. A plurality of spacecrafts coupled to an assist module are injected into an interplanetary orbit having a periodicity synchronous with the earth&#39;s revolution period. Then, in a maneuver for allowing the assist module to re-counter with and pass near to the earth Earth swing-by), the assist module appropriately performs an orbital change maneuver and a separation maneuver for each of the spacecrafts in a sequential order. Through these maneuvers, a closest-approach point to the earth is changed for each of the spacecrafts so as to drastically change a subsequent orbital element for each of the spacecrafts. The assist module takes a sufficient time to determine a target orbit for each of the spacecrafts with a high degree of accuracy until a half month to several days before a closest-approach time in the Earth swing-by. Based on the determined orbit, the assist module makes an orbit correction of about several m/sec in a sequential order, and then separates the spacecrafts therefrom in a sequential order. In this process, an inertia navigation is performed based on an accelerometer mounted in the assist module and information about attitude.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to a space system including a single assist module coupled to a plurality of space-vehicles or spacecrafts, and more particularly to a method of launching the single assist module together with the plurality of spacecrafts and then controllably guiding the spacecrafts into different desired orbits, such as interplanetary orbits, individually. 2. DESCRIPTION OF THE BACKGROUND ART

A problem to be solved by the present invention is a difficulty in, after launching a plurality of spacecrafts all together, injecting them into different orbits, such as circum-earth orbits or interplanetary orbits, individually. A conventional solution of this problem has involved the need for equipping each of the spacecrafts with its own rocket (launch vehicle), and inevitably led to significant increase in cost. Particularly, in an operation for injecting a plurality small-size spacecrafts into a variety of different orbits individually, it has been essentially required to equip each of the spacecrafts with its own launching means. Thus, such an operation has been regarded as unrealizable because of extremely poor efficiency and unacceptable cost performance.

Japanese Patent Laid-Open Publication No. 07-187091 (Patent Publication 1) discloses a method of launching a plurality of space satellites, particularly, a technique of reducing a fuel consumption in an orbit-control propulsion system (orbital maneuvering system) for orbital transfer.

In this technique disclosed in the Patent Publication 1, two satellites S1, S2 mounted on a single common rocket and launched simultaneously are injected into a parking orbit P having a semi-major axis length less than those of first and second target orbits A1, A2. Then, when the parking orbit P comes close to being in plane with the first target orbit A1, an in-plane orbit control is performed using a propulsion system for the first satellite S1 to move the first satellite S1 to the first target orbits A1 via a transfer orbit A1′to the first target orbits A1. Then, in the same manner, when the parking orbit P comes close to being in plane with the second target orbit A2, the second satellite S2 is moved to the second target orbits A2 via a transfer orbit A2′ to the second target orbits A2.

Fundamentally, the technique disclosed in the Patent Publication 1 is not designed to change an inclination of a circum-earth orbit. In contrast, the present invention is premised on a system capable of changing an orbital element by means of a perturbation effect arising from a gravitational field of the earth.

In addition to the above problem in principle about incapability of maneuvering an inclination of an established orbit, the technique disclosed in the Patent Publication 1 is not originally designed to inject a plurality of spacecrafts into different interplanetary orbits individually.

SUMMARY OF THE INVENTION

In view of the above circumstances, the present invention provides a method for injecting a plurality of spacecrafts escaping from a gravitational sphere of the earth, into at least two different orbits in a single launch and then loosing the plurality of spacecrafts, respectively, toward different targets. The method comprises the steps of injecting the plurality of spacecrafts into an interplanetary orbit having a periodicity synchronous with the earth's revolution period, in a state of being coupled to a single assist module for assisting a space flight thereof, and performing a swing-by maneuver relative to the earth, wherein, in advance of executing the swing-by maneuver, the orbit of the assist module is changed in a sequential order with respect to each of the plurality of spacecrafts.

In one preferred aspect of the present invention, the step of performing a swing-by maneuver relative to the earth includes changing the orbit of the assist module in a sequential order with respect to each of the plurality of spacecrafts a half month to several days before executing the swing-by maneuver, so as to control a closest-approach point to the earth for each of the spacecrafts in conformity to a target orbit for each of the spacecrafts, and then separating each of the spacecrafts from the assist module in a sequential order.

In another aspect of the present invention, the method includes the step of, when each of the spacecrafts after being separated from the assist module in the swing-by step passes through the closest-approach point to the earth, subjecting the spacecraft to an acceleration/deceleration control unique to the spacecraft. In this case, when the spacecraft is subjected to the deceleration control, an established orbit will be an elliptical orbit around the earth.

Preferably, the interplanetary orbit having a periodicity synchronous with the earth's revolution period is one selected from the group consisting of a plurality of singular trajectories or orbits around the sun each of which has a synchronous period including six months, one year, an integer number of years and a period having a rational or irrational number ratio with respect to the earth's revolution period, and a property of being able to re-encounter with the earth regardless of a velocity of escape from the earth.

The term “singular trajectory” means an orbit around the sun which has an irrational period of about 1.4 years, 2.4 years, -----, and a property of being able to re-encounter with the earth regardless of a velocity of escape from the earth.

In another preferred aspect of the present the step of performing a swing-by maneuver relative to the earth includes, after executing the swing-by maneuver, further repeatedly revolving the assist module around the sun, and executing an additional swing-by maneuver similar to the swing-by maneuver as a backup action for loosing the spacecrafts, respectively, toward specific interplanetary orbits.

The step of performing a swing-by maneuver relative to the earth may include a deceleration maneuver for allowing the spacecrafts to be captured by the earth's gravitational sphere and injected into at least two different elliptical orbits around the earth.

In this case, the at least two different elliptical orbits may be selected from the group consisting of a combination of at least two elliptical orbits each having a different inclination, a combination of at least two direct and retrograde elliptical orbits, and a combination of at least two polar and low-inclination elliptical orbits.

The swing-by maneuver relative to the earth may be performed by use of the gravity of a planet other than the earth.

The step of injecting the plurality of spacecrafts into an interplanetary orbit having a periodicity synchronous with the earth's revolution period may be initiated in a state when the plurality of spacecrafts coupled to the assist module makes a flight in a circumterrestrial space or an interplanetary space.

As above, the method of the present invention makes it possible to inject a plurality of spacecrafts, i.e., at least two spacecrafts or, if needed, dozens of spacecrafts, into different interplanetary orbits or different orbits around the earth (circum-earth orbits) individually in a single launch using the single assist module.

When the spacecraft is a planetary exploration spacecraft, the method of the present invention makes it possible to selectively determine a plurality of different directions escaping from the earth, in a single launch and loose a plurality of planetary exploration spacecrafts toward different astronomical objects individually. Generally, in a launch of a rocket or the like, an escape direction from an orbit of a spacecraft mounted on the rocket or the like is limited to only an in-plane direction of a parking orbit of the rocket or the like. The method of the present invention can completely remove this restriction. In particular, when the planetary exploration spacecraft is a small-size payload, the method of the present invention makes it possible to loose dozens of payloads toward different targets individually. Thus, the method of the present invention can be widely applied to an observation mission of a small celestial body, such as near-earth miniplanet or minicomet.

When the spacecraft is a space satellite, a plurality of launches have been required to inject a plurality of satellites into different orbits, particularly, largely different in inclination. The method of the present invention makes it possible to inject a plurality of satellites into direct and retrograde elliptical orbits, or polar and low-inclination elliptical orbits in a single launch. Thus, the method of the present invention can be expected to have wide applications to a satellite system (constellation) achieving its function using a navigation, communication or global positioning satellite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an assist module mounting a plurality of spacecrafts.

FIG. 2 is a schematic diagram showing one example of the spacecraft.

FIG. 3 is a conceptual diagram showing an orbit injection represented using an inertial coordinate-system on the assumption that a synchronous period is one year.

FIG. 4 is a conceptual diagram showing an orbit injection represented using a rotating coordinate system wherein a line between the sun and the earth is fixed.

FIG. 5 is a conceptual diagram showing a maneuver in an orbital plane.

FIG. 6 a conceptual diagram showing a technique of combining a plurality of maneuvers in an out-of-plane direction of an orbit to achieve a plurality of different orbits.

FIG. 7 is a graph showing one example of a synchronous orbit having an encounter period of one year, for use in injecting four spacecrafts into four different target interplanetary orbits in a single launch using a single assist module.

FIG. 8 is a graph showing a time of separating each spacecraft from an assist module and a control quantity of the separation maneuver.

FIG. 9 is a graph showing a state of a target miniplanet-flyby after an Earth swing-by in each spacecraft separated in a process illustrated in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

All spacecrafts to be injected into different orbits are coupled to an assist base module, and injected into a parking orbit, which is an interplanetary orbit having the shortest periodicity synchronous with the earth's revolution period, in a single launch. The assist base module plays two rolls in correcting an initial orbit-injection error and in finely changing its own orbit in advance of execution of a swing-by maneuver relative to the earth (Earth swing-by) and then separating each of the mounted spacecrafts therefrom in a sequential order.

In view of further reliable mission accomplishment, after the Earth swing-by, it is desirable to optionally take a backup action of further repeatedly revolving the assist base module around the sun.

While there is an orbit having a significantly short period, such as 0.5 years, as the shortest synchronous orbit, a synchronous orbit having an encounter period of one year may be practically used, in view of a launch or transport capability of launching means required for an orbit injection of the assist base module.

With reference to the drawings, an embodiment of the present invention will now be specifically described.

This embodiment is a method for injecting four spacecrafts consisting of a first spacecrafts 1, a second spacecraft 2, a third spacecraft 3 and a fourth spacecraft 4, into four different target interplanetary orbits in a single launch using a single assist module 5 for the purpose of exploration of four different miniplanets (name: 1999 NA5, 2002 AY1, 2003 YN1 and 2004 MO3). FIG. I shows the four spacecrafts 1, 2, 3, 4 mounted on the assist module 5. In this embodiment, the four cylindrical-shaped spacecrafts are mounted on a circular top surface of the cylindrical-shaped assist module 5 in a concentric arrangement. The assist module 5 is provided with a plurality of attitude-control thrusters 6 at a side surface thereof. The assist module 5 is further provided with a propulsion thruster (not shown) adapted to emit a jet in an axial direction of the spacecrafts, at a bottom surface thereof The following functions is required, but not limited thereto, for the assist module 5:

-   -   1. Function of orbit control;     -   2. Function of communication with ground stations;     -   3. Function of attitude control (including function of attitude         determination); and     -   4: Function of mechanical holding and separation of each         spacecraft.

FIG. 2 schematically shows one example of the spacecraft to injected into a given target interplanetary orbit in a state of being mounted on the assist module. The spacecraft in this example is provided with a CCD camera 7 for imaging and orbit estimation, a fish-eye lens 8 for imaging, a near-infrared spectroscope 9 for spectrum observation, a 2-dimensional sun sensor 10 for attitude detection, an RF sensor 11 for attitude detection, an ultra-stable oscillator (USO) or transponder 12 for distance measurement, an omnidirectional antenna 13 for communication, a new-type cold gas jet 14 for orbit and attitude control, a data recorder (DR) 15 for data storage and a battery (BAT) 16 for power supply.

In the method of the present invention, a plurality of spacecrafts coupled to a single assist module is injected into an interplanetary orbit having a periodicity synchronous with the earth's revolution period as described above, and then a maneuver for allowing the assist module with the spacecrafts to re-encounter with the earth and pass through the vicinity of the earth (hereinafter referred to as “Earth swing-by”) is performed. During the Earth swing-by, the assist module appropriately performs an orbital change maneuver and a separation maneuver for each of the spacecrafts in a sequential order. Through these maneuvers, a closest-approach point to the earth is changed for each of the spacecrafts so as to drastically change a subsequent orbital element for each of the spacecrafts. FIG. 3 shows a concept of this orbit injection represented using an inertial coordinate-system. While FIG. 3 shows one example where a synchronous period is one year, various synchronous periods may be selected. FIG. 4 shows this concept represented using a rotating coordinate system wherein a line between the sun and the earth is fixed. FIGS. 5 and 6 show a plurality of orbits during the Earth swing-by.

FIG. 5 shows a maneuver in an orbital plane. FIG. 6 shows one example a plurality of different orbits to be achieved by combining a plurality of maneuvers in an out-of-plane direction of an orbit. The assist module takes a sufficient time to determine a target orbit for each of the spacecrafts with a high degree of accuracy until a half month to several days before a closest-approach time in the Earth swing-by. Based on the determined orbit, the assist module makes an orbit correction of about several m/sec in a sequential order, and then separates the spacecrafts therefrom in a sequential order. In this process, an inertia navigation is performed based on an accelerometer mounted in the assist module and information about attitude to adequately control an orbital quantity.

While the level of change in escape direction is largely varied depending an intended purpose of each of the spacecrafts, an escape direction can be changed in an angle of one radian by changing the closest-approach point in a distance of about 10,000 km during the Earth swing-by. In this case, the level of the orbit correction to be performed by the assist module is no more than 1 to 10 m/sec although it is varied depending on the timing of implementation thereof If all directions of a plurality of escape orbits can be assured within several degrees, the orbit correction to be performed by the assist module is about 1 m/sec.

While the orbit correction to be performed by the assist module can include an operational error, the level of the error is about 1 to 2 cm/sec in a practical sense and in an empiric sense, and an orbit correction after the Earth swing-by can be confined to I to 10 m/sec. These control quantities are determined with respect to each target interplanetary orbit, and the assist module is controlled based on the determined control quantities at a given timing according to a remote command from the earth.

FIG. 7 illustrates one example of a synchronous orbit having an encounter period of one year, for use in injecting the four spacecrafts 1, 2, 3, 4 into four different target interplanetary orbits in a single launch using the single assist module 5. During the course from a launch to an Earth swing-by in this example, the assist module 5 mounting the four spacecrafts 1, 2, 3, 4 is injected into a synchronous orbit to perform the Earth swing-by one year from the launch.

A launch time in this example is Jul/02/2009, and launch conditions include an extra-velocity squared C3 =9.0 km/s², an escape asymptote declination of- 26.0° and an escape asymptote right-ascension of 99.7°. Further, a time of re-encounter with the earth is one year from the launch (Jul/02/2010).

FIG. 8 shows a time of separating each of the spacecrafts 1, 2, 3, 4 from the assist module 5 and a control quantity of the separation maneuver. That is, FIG. 8 shows a control quantity and time of a pre-Earth swing-by targeting. The pre-Earth swing-by targeting in this example is performed on a target plane (B-plane), and the four spacecrafts 1, 2, 3, 6 are separated at the following times and in the following control quantities:

-   -   Jun/19/2010 Separation of the first spacecraft 1     -   Jun/20/2010 First targeting maneuver (3.1 m/s)         -   Separation of the second spacecraft 2     -   Jun/21/2010 Second targeting maneuver (6.8 m/s)         -   Separation of the third spacecraft 3     -   Jun/22/2010 Third targeting maneuver (1.2 m/s)         -   Separation of the fourth spacecraft 4

FIG. 9 shows a state of a target miniplanet-flyby (passing near to each target miniplanet) after the Earth swing-by in each of the spacecrafts 1, 2, 3, 4 separated in the above process.

As seen in FIG. 9, the spacecrafts 1 to 4 fly, respectively, in different orbits and toward different target miniplanets.

A relationship of target miniplanet-flyby times of the spacecrafts and the corresponding target miniplanets is as follows: Jul/01/2010 Earth swing-by Dec/21/2010 The second spacecraft 2 The miniplanet 2002 AY1-flyby Jan/04/2011 The fourth spacecraft 4 The miniplanet 2004 MO3-flyby Feb/08/2011 The third spacecraft 3 The miniplanet 2003 YN107-flyby Apr/11/2011 The first spacecraft 1 The miniplanet 1999 NA5-flyby

The above control and the conditions of the encounter between each of the spacecrafts and each of the target miniplanets are summarized as follows: Spacecraft first second third fourth spacecraft spacecraft spacecraft spacecraft Miniplanet 1999 NA5 2002 AY1 2003 YN107 2004 MO3 Flyby Date Apr/11/2011 Oct/21/2010 Feb/08/2011 Jan/04/2011 Relative 4.7 km/s 18.3 km/s 1.7 km/s 11.1 km/s Velocity between Spacecraft and Miniplanet Relative 59.7° 48.2° 18.7° 11.9° Velocity/ Sun Angle Distance from 0.15 AU 0.33 AU 0.32 AU 0.33 AU Earth Absolute 20.3 20.6 26.2 21.2 Magnitude

The relative velocity/sun angle is an angle between a sun direction and a vector direction viewed from each of the spacecrafts.

INDUSTRIAL APPLICABILITY

The international acquisition of a patent right on this invention will bring about positive effects in achievement of international cooperation-based missions. The interplanetary-orbit injection method of the present invention is strongly desired by planetary science laboratories in many countries. The present invention would contribute to space science, particularly, planetary science research. 

1. A method for injecting a plurality of spacecrafts escaping from a gravitational sphere of the earth, into at least two different orbits in a single launch and then loosing said plurality of spacecrafts, respectively, toward different targets, comprising the steps of: injecting said plurality of spacecrafts into an interplanetary orbit having a periodicity synchronous with the earth's revolution period, in a state of being coupled to a single assist module for assisting a space flight thereof; and performing a swing-by maneuver relative to the earth, wherein, in advance of executing said swing-by maneuver, the orbit of said assist module is changed in a sequential order with respect to each of said plurality of spacecrafts.
 2. The method as defined in claim 1, wherein said step of performing a swing-by maneuver relative to the earth includes changing the orbit of said assist module in a sequential order with respect to each of said plurality of spacecrafts a half month to several days before executing said swing-by maneuver, so as to control a closest-approach point to the earth for each of said spacecrafts in conformity to a target orbit for each of said spacecrafts, and then separating each of said spacecrafts from said assist module in a sequential order.
 3. The method as defined in claim 1, which includes the step of, when each of the spacecrafts after being separated from said assist module in said swing-by step passes through said closest-approach point to the earth, subjecting said spacecraft to an acceleration/deceleration control unique to said spacecraft.
 4. The method as defined in either one of claim 1, wherein said interplanetary orbit having a periodicity synchronous with the earth's revolution period is one selected from the group consisting of a plurality of singular trajectories or orbits around the sun each of which has a synchronous period including six months, one year, an integer number of years and a period having a rational or irrational number ratio with respect to the earth's revolution period, and a property of being able to re-encounter with the earth regardless of a velocity of escape from the earth.
 5. The method as defined in either one of claim 1, wherein said step of performing a swing-by maneuver relative to the earth includes, after executing said swing-by maneuver, further repeatedly revolving said assist module around the sun, and executing an additional swing-by maneuver similar to said swing-by maneuver as a backup action for loosing said spacecrafts, respectively, toward specific interplanetary orbits.
 6. The method as defined in either one of claim 1, wherein said step of performing a swing-by maneuver relative to the earth includes a deceleration maneuver for allowing said spacecrafts to be captured by the earth's gravitational sphere and injected into at least two different elliptical orbits around the earth.
 7. The method as defined in claim 6, wherein said at least two different elliptical orbits are selected from the group consisting of a combination of at least two elliptical orbits each having a different inclination, a combination of at least two direct and retrograde elliptical orbits, and a combination of at least two polar and low-inclination elliptical orbits.
 8. The method as defined in either one of claim 1, wherein said swing-by maneuver relative to the earth is performed by use of the gravity of a planet other than the earth.
 9. The method as defined in either one of claim 1, wherein said step of injecting said plurality of spacecrafts into an interplanetary orbit having a periodicity synchronous with the earth's revolution period is initiated in a state when said plurality of spacecrafts coupled to said assist module makes a flight in a circumterrestrial space or an interplanetary space. 